Research Article

Chronic TLR7 and TLR9 signaling drives anemia via differentiation of specialized hemophagocytes

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Science  11 Jan 2019:
Vol. 363, Issue 6423, eaao5213
DOI: 10.1126/science.aao5213

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Unmasking an agent of inflammatory anemia

Infectious and autoimmune diseases are associated with anemia and thrombocytopenia. A severe form of inflammatory cytopenia called macrophage activation syndrome (MAS) may occur during rheumatological disorders and viral infections. Akilesh et al. show that monocyte recognition of self- or pathogen-derived nucleic acids via Toll-like receptors 7 and 9 (TLR7 and TLR9) drives MAS-like disease in mice. TLR7 or TLR9 signaling in monocytes causes these cells to differentiate into inflammatory hematophagocytes (iHPCs), which are similar to but distinct from red pulp macrophages. Preventing iHPC differentiation by depleting monocytes relieves MAS-like symptoms. When mice were subjected to a model of malarial anemia, MyD88- and endosomal TLR-dependent iHPC differentiation also occurred. Thus, iHPCs may play a role in both MAS-driven and malarial anemia, as well as thrombocytopenia.

Science, this issue p. eaao5213

Structured Abstract

INTRODUCTION

Inflammatory disorders and infections are associated with cytopenias, including anemia and thrombocytopenia. Common to these conditions is activation of the innate immune system, which includes the monocyte/macrophage lineage, through receptors that recognize pathogen-derived molecules. Toll-like receptors (TLRs) are one family of pathogen sensors that recognize bacterial and viral ligands, including pathogen-derived nucleic acids through TLR7 and TLR9. TLRs not only recognize pathogens, leading to clearance of infection, they are also implicated in inflammatory and autoimmune disease. How signaling through TLRs in monocytes and/or macrophages may participate in inflammation- and infection-associated cytopenias is not completely understood.

RATIONALE

The phagocytosis of red blood cells (RBCs), platelets, and leukocytes can be a major contributor to acute cytopenias. Thus, we reasoned that specialized phagocytes may develop in inflammatory conditions in response to the signaling of pattern-recognition receptors, such as TLRs. In vitro, TLR signaling can directly induce macrophage development. However, whether TLR-induced differentiation specifies a particular macrophage fate that is distinct from homeostatic macrophage differentiation is not clear. To address this question, we undertook in vitro and in vivo studies to investigate TLR-induced macrophage differentiation and the role of this process in inflammatory cytopenias.

RESULTS

Transcriptional profiling of macrophages differentiated through TLR7 signaling in vitro showed that these cells have a gene signature similar to red pulp macrophages (RPMs), the steady-state hemophagocytes of the spleen. Using a mouse model of TLR7-driven inflammation (the TLR7.1 mouse), we found a population of hemophagocytes that were distinct from RPMs by cell-surface phenotype and that were not found in uninflamed wild-type mice. Therefore, we have termed these cells “inflammatory hemophagocytes,” or iHPCs. iHPCs shared expression of the transcription factor Spi-C with RPMs but showed higher phagocytic uptake of RBCs. Thus, iHPCs, although similar to RPMs, are a distinct population of hemophagocytes.

iHPCs required cell-intrinsic TLR7 signaling for their development and differentiated from inflammatory Ly6Chi monocytes by way of IRF5. Signaling in these monocytes through TLR7 or TLR9—but not through TLR4, TLR3, interleukin-1β, or interferon-γ—induced the iHPC phenotype, and chronic depletion of monocytes caused a severe reduction in iHPC numbers. TLR7.1 mice showed progressive severe anemia and thrombocytopenia with age, and blocking iHPC differentiation through monocyte depletion rescued these inflammatory cytopenias. We next asked whether infection-driven anemia and thrombocytopenia also involved iHPCs. Infection of mice with RBC stage Plasmodium yoelii 17XNL, a model of malarial anemia, showed MyD88 and endosomal TLR-dependent iHPC differentiation. Thus, iHPCs differentiate in situations of both sterile and infectious cytopenias.

CONCLUSION

We identified a previously unknown TLR7/9-driven monocyte differentiation pathway associated with inflammatory cytopenias. We propose that macrophage activation syndrome (MAS), a life-threatening complication of rheumatological diseases or viral infection associated with acute cytopenias, may be caused by iHPCs and that these cells also drive severe malarial anemia. Our studies suggest unexplored avenues to treat MAS, severe malarial anemia, and “anemia of inflammation.”

iHPCs differentiate in response to TLR7 and TLR9 signals and participate in inflammation-induced anemia.

(A) Before release into the circulation, Ly6Chi monocytes develop in the bone marrow from hematopoietic stem cells (HSCs) and common myeloid progenitors (CMPs). During inflammation, including TLR7 overexpression and Plasmodium infection, Ly6Chi monocytes differentiate into hemophagocytes in response to signaling from the endosomal nucleic acid–sensing receptors TLR7 and TLR9 by means of the IRF5 transcription factor. These splenic iHPCs phagocytose RBCs and express the transcription factor Spi-C and cell-surface proteins CD31 and DR3. (B) iHPCs drive inflammatory anemia and thrombocytopenia, such as that seen in MAS and severe malarial anemia.

Abstract

Cytopenias are an important clinical problem associated with inflammatory disease and infection. We show that specialized phagocytes that internalize red blood cells develop in Toll-like receptor 7 (TLR7)–driven inflammation. TLR7 signaling caused the development of inflammatory hemophagocytes (iHPCs), which resemble splenic red pulp macrophages but are a distinct population derived from Ly6Chi monocytes. iHPCs were responsible for anemia and thrombocytopenia in TLR7-overexpressing mice, which have a macrophage activation syndrome (MAS)–like disease. Interferon regulatory factor 5 (IRF5), associated with MAS, participated in TLR7-driven iHPC differentiation. We also found iHPCs during experimental malarial anemia, in which they required endosomal TLR and MyD88 signaling for differentiation. Our findings uncover a mechanism by which TLR7 and TLR9 specify monocyte fate and identify a specialized population of phagocytes responsible for anemia and thrombocytopenia associated with inflammation and infection.

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